Ethylene/Aromatic Vinyl Compound (Styrene) Copolymers
Some ethylene/aromatic vinyl compound (styrene) random copolymers obtainable by means of a so-called homogeneous Ziegler-Natta catalyst system comprising a transition metal catalyst component and an organic aluminum compound, and processes for their production, are known.
JP-A-3-163088 and JP-A-7-53618 disclose ethylene/styrene copolymers having a styrene content of at most 50 mol % and containing regular (i.e. head-to-tail) styrene chain, so-called pseudo-random copolymers, obtainable by means of a complex having a so-called constrained geometric structure.
JP-A-6-49132 and Polymer Preprints, Japan, 42, 2292 (1993) disclose processes for producing similar ethylene/styrene copolymers having an aromatic vinyl compound content of at most 50 mol % and containing regular aromatic vinyl compound chain, so-called pseudo-random copolymers, by means of a catalyst comprising a bridged metallocene type Zr complex and a cocatalyst. These copolymers have no stereoregularity derived from aromatic vinyl compound units.
Further, recently, it has been reported to produce an ethylene/aromatic vinyl compound copolymer having a stereoregularity of alternating copolymerization type by means of a certain specific bridged bisindenyl type Zr complex i.e. a racemic[ethylenebis(indenyl)zirconium dichloride] under an extremely low temperature (−25° C.) condition. (Macromol. Chem., Rapid Commun., 17, 745 (1996).) However, with the copolymer obtainable by this complex, the molecular weight is not yet practically sufficient, and the compositional distribution is also large.
Further, JP-A-9-309925 and JP-A-11-130808 disclose novel high-molecular weight ethylene/styrene copolymers which respectively have styrene contents of from 1 to 55 mol % and from 1 to 99 mol % and which have ethylene/styrene alternating structures and isotactic stereoregularity in their styrene chain structures and further have head-to-tail styrene chain structures, with the alternating degrees (λ values in this specification) of the copolymers being at most 70. Further, these copolymers have relatively high transparency.
However, it has been understood that transparency of these copolymers decreases if they have a certain extent of compositional distribution. If transparency decreases, application to use for which high transparency is required, such as alternative to soft vinyl chloride resins is difficult in some cases.
Such copolymers have a relatively high melting point (heat resistance) at a low styrene content compositional region, but have high hardness, and they will hardly be employed for use as soft resins. Hardness can be decreased by increasing the styrene content, but the melting point decreases, and heat resistance will be impaired.
As mentioned above, conventional ethylene/styrene copolymers can hardly satisfy heat resistance and flexibility simultaneously.
The present invention is to overcome the above problems of conventional technique, and to provide a process for producing an olefin/aromatic vinyl compound copolymer which is excellent in transparency and which satisfies flexibility and heat resistance simultaneously with a practically high productivity.